from adrenalectomy, or from any situation in- 
volving chronic disturbing situations. No wild 
rats survived following adrenalectomy accompanied 
by salt therapy, whereas most domesticated rats 
survived. When placed in a situation demanding 
choice among a battery of new types of food stuffs, 
the wild rats refused the strange food, lost weight 
and died, whereas the domesticated rats made the 
proper selections and lived. Under conditions of 
temporary acute stress (low temperature, loud 
noise, or fighting) there appeared to have been 
little change in the adrenal, as judged by ascorbic 
acid content, in wild rats but marked change in 
domesticated strains {84). On the growth side 
administration of growth hormone to domesticated 
rats regularly produces giganticism, whereas in 
other animals such as the dog it results in diabetes 
{85, 86, 87). To my knowledge no one has sub- 
jected wild rats to chronic injections of growth 
hormone. A distinct possibility of diabetes is 
indicated. 
7 . Mortality in Rat Populations 
If we begin with a group of rats at weaning and 
follow them as they grow older we note that death 
gradually exerts an attrition on its members. 
Surprisingly enough few populations of rats have 
so been followed. 
In addition to the data of the present study of the 
Towson population, there are available in the 
literature three other studies of survival among 
populations of rats. These will be examined to 
determine some of the major factors affecting 
survival. The four populations in order of de- 
creasing severity of environmental conditions are: 
1. Davis {88). Wild Norway rats on a farm 
where food and shelter was apparently in 
abundance but where heavy predation from 
owls, cats, and dogs existed in addition to the 
bacterial disease, Salmonella. Free play of 
social interaction also presumably occurred. 
2. Calhoun — present study. Wild Norway rats 
maintained in an enclosure with excess food, 
absence of predators, but presence of certain 
diseases such as bronchiectasis and helminth 
parasites, and with opportunity for full ex- 
pression of social interaction. 
3. King {64, 63). A wild type of Norway rat 
maintained in the laboratory for 25 genera- 
tions after capture from the wild state. The 
survival data refers to this entire period. 
Rats were maintained in large cages (see 
further comment on pp. 262 to 264) in groups 
of 5-7 related individuals. This permitted a 
certain amount of social conflict but preda- 
tion was absent and disease was presumed to 
be at a low level. Only rats belonging to 
litters all of whose members survived to 60 
days of age were included in this study. 
4. Wiesner and Sheard {65). Wistar albino 
strain of Norway rats. These were main- 
tained in small cages with one male and one 
female and their unweaned young. All 
factors contributing to mortality including 
complex social interaction were at a mini- 
mum. 
The data for each of these populations were 
recalculated in terms of proportion of survivors 
dying each 50 days. These proportions repre- 
sent age specific mortality rates. Lines approxi- 
mating the calculated points were fitted by eye 
(figs. 153 and 154). Inspection of the trends of 
change of mortality rate indicate that they are of 
two types: those in which mortality rate decreases 
with age and those in which it increases with age. 
A. Decreasing mortality rate with age. Three 
(Davis’ males, Calhoun’s males and females) of 
the eight sets of data exhibit decreasing mortality 
rates with increasing age over an initial portion 
of the life span. From the statistics alone we 
cannot gain further insight into the origin of these 
trends. But interestingly enough they only charac- 
terize populations inhabiting environments, com- 
plex with regard to both their social and physical 
aspects. My own observations indicate that rats 
just weaned in the complex environment of the 
free ranging state have much to learn. Each 
unlearned aspect of the environment places the 
rat at a disadvantage with reference to his older 
associates. We can, therefore, suspect that it is 
such a learning to cope with the conditions and 
exigencies of the environment, which for a time 
following weaning, continually reduces the pro- 
portion of survivors which die. And yet in each 
of these three cases there arrives an age when the 
advantages accruing to the learning process were 
no longer adequate to compensate for the debilities 
of the ongoing aging process which increase 
the probability of death at each successive age. 
A close parallel to the above phenomenon 
characterizes the changing death rate due to motor 
vehicle accidents for humans. As may be seen in 
figure 8.1 of the vital statistics for the United States 
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